my son is 6 and i really only check him to make sure he's rinsed well. he will either take a bath or a shower.
i would say around 4-5 i started being in there less...but we have a small duplex (2 bdrm) that basically, the bathroom is right in the center of. i literally can't go into any room without being in earshot, and within about 15 feet of him lol. it's just what you're comfortable with. we would do a lot of, "doing ok?" "yes mommy!" lol.
honestly, it sounds more like you are entertaining her rather than "keeping an eye on her". i got tired of that quick too. you aren't obligated to entertain her. start scaling back a bit, no problem with that.
also, if daddy is uncomfortable (or bored lol), she doesn't HAVE to take an hour long bath. he can tell her, okay, we're going to do a bath but we're going to see how fast we can do it, so we can get back to reading/playing/whatever. she might not be thrilled not to have her playtime, but he's in charge. if he says short bath, short bath.